Stabilization of borax supersaturation in solutions



March 19, 1957 s. TAYLOR ET AL 2,785,952

STABILIZATION OF BORAX SUPERSATURATION IN SOLUTIONS Original Filed April10, 1953 Ell-IL...) A

SYMBOLS 0 E2504 (Ni/4), sw m/20 o P2304 4.2 H20 A Fc/ 224,0

18$. BORAX CRYSTALLIZED FeaM 1001.86. Sou/710M nv 24 Hoe/Es E o :o-u-rx000 0.05 0.10 0.15 0.20

Wr. Penceur Fe, o- Dev .Tucksolsurs DONALD 3. Timon,

GEORGE A. CONNELL,

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United States Patent STABILIZATION or BORAX SUPERSATURATION IN SOLUTIONSDonald S. Taylor, Whittier, and George A. Connell, Los Angeles, Calif.,assignors, by mesne assignments, to United States Borax & ChemicalCorporation Original application April 10,1953, Serial No. 347,892.Divided and this application February 13, 1956, Serial No. 565,066

2 Claims. (CI. 2359) This invention has to do generally with thestabilization of solutions that are supersaturated with respect toborax.

The phenomenon of supersaturation is particularly common in the generalsystem Na2OB2OaH2O, with or without other solutes. A metastable state ofsupersaturation is frequently encountered, in which an excess of boratewill stay in solution almost indefinitely in the absence of appropriateseed crystals. In some cases even in the presence of seed crystals thesupersaturation is quite persistent, and breaks relatively slowly.

It has been discovered that certain substances, when present insolutions supersaturated with respect to borax, have a strong tendencyto stabilize the state of ,supersaturation. Such stabilizing agentspermit a more extremc degree of supersaturation to be effectivelyattained and utilized, other conditions being the same; and they mayextend almost indefinitely a condition of supersaturation that wouldotherwise be relatively short lived.

The most marked effect, and in many respects the most frequently useful,is the change that such agents produce in the behavior of moderatelysupersaturated solutions in the presence of seed crystals. In manyinstances growth of seed crystals in such solutions is substantiallycompletely inhibited, even at concentrations that would normally producefairly rapid precipitation.

The substances that have been found to have this remark'able property ofstabilizing solutions that are supersaturated with respect to boraxinclude ferric iron, which is most readily introduced into solutions ofborates by dissolving a ferrous salt and providing in the solution anoxidizing agent capable of oxidizing the ferrous ion to ferric ion.Under many practical conditions dissolved atmospheric oxygen is aneffective oxidizing agent. Or any suitable oxidizing agent may be added,such, for example, as potassium dichromate, potassium permanganate andthe like. The ferric iron may be in the form of free ferric ion, orcombined as ferric hydroxide, de-

pending primarily upon thepH of the solution. At relatively high pH thesolubility of ferric hydroxide is extremely low in absence of othersolutes, and is quite low even in concentrated solutions of borates.Considerable stabilization is still obtained under those conditions, andis probably caused primarily either by the smallconcentration of ferrichydroxide in solution or by the very fine ferric hydroxide precipitate.

Aluminum, cupric, chromic and, to a lesser extent, zinc, calcium andmagnesium ions also act as stabilizing agents for excess borax insolution.

In addition to the inorganic substances mentioned, anionic organicsubstances having surface active properties have been found to beeffective stabilizing agents. Such surface active substances aredescribed and claimed in our copending patent application, Serial No.117,806, filed September 26, 1949, now Patent 2,662,810. The presentapplication is a division of our copending application, Serial No.347,892, filed April 19,. 1953, ingw 2,785,952 Patented Mar. 19, 19 57 2Patent No. 2,774,070, which is a continuation-in-part of Serial No.117,806.

An example of the practical utilization of the invention is provided bythe problem of preparing solutions of' sodium borate for use incontrolling weeds. Such solu-- tionsare ordinarily made up as neededfrom solid compositions. It is desirable in general to employ aconcentration of borate in solution that is close to, or even somewhatabove, the normal solubility, in order to apply a maximum quantity ofborate per pound of solution. By providing a stabilizing agent insuitable quantity in the solution, it becomes feasible to employsolutions that are very appreciably supersaturated with respect toborax.. In that way the invention permits applying a given quantity ofborate more cheaply and conveniently, since less water is consumed andless bulk of solution needs to be handled.

That procedure is particularly useful when the borate composition is ofthe type described and claimed in the copending patent of application ofGeorge A. Connell entitled Supersaturated Borax Solutions and Methodsand Compositions for Producing Same, Serial No. 117,- 878, filedSeptember 26, 1949, now Patent 2,643,947. As is there described in fulldetail, it has been discovered that a mixture of sodium tetraborate andboric acid (or sodium pentaborate) having a suitable value of the molarratio of NazO/BzOs can be dissolveddirectly in water to yield asupersaturated solution. The concentration obtainable may be equal to orgreater than the maximum of the pentaborate peak of the normalsolubility curve, which occurs typically at a value of the sodium toboron ratio of from about 0.15 to about 0.25, depending upon conditionssuch as temperature and presence of other solutes.

An important feature of that'discovery is that the actual sodium toboron ratio of the ingredients dissolved (and of the resulting solution)need not coincide with the ratio at the peak of the solubility curve,but can be considerably higher, and the concentration of the resultingsolution may still be equal to or greater than that at the maximum ofthe curve. Such direct solution of sodium tetraborate to produce a.supersaturated solution can be obtained by using tetraborate containingfive mols of water of hydration or less.

An advantage of that discovery in preparing such solutions for Weedcontrol purposes is that the variation with temperature of the sodium toboron ratio at which the solubility peak occurs can be largelyneglected. A dry composition of the type described suitable for solutionin water at one temperature can thus be dissolved directly to the sameconcentration even in considerably colder water. Under the latterconditions a supersaturated solution ordinarily results. The practicaladvantage of being able to stabilize that supersaturation by addition ofa simple and relatively inexpensive stabilizing agent in accordance withthe present invention is obvious.

The amount of any particular stabilizing agent that should be used inany given instance depends upon the conditions of use, for example thelength of time that supersaturation must be effectively maintained; anddepends particularly upon the degree of the supersaturation. The latterfactor is difiicult to define usefully in precise terms, because of thewide variation of the normal solubility as a function of the ratio ofNazOs to B203 in the solution. Because of the nature of that variation,the tendency of borax to crystallize from a supersaturated solutiondepends not only upon the percentage excess of B203 in solution, butalso upon the value of the ratio NazO/BaOs and the behavior of thesolubility curve in that region. The pentaborate peak of the normalsolu-t 3 bility curve is our. sxaai ls o such rapid ariatisn Qsolubility with variation of the NazO/BzOs ratio.

Many of the agents mentioned exhibit a very considerab e s abil zingettestw e sed n a o n r ti n o 0- uniform cOnditions to be described,are illustrated graphically in the drawing. The solutions were made upby dissol ing 1.5 lbs. of a dry mix per gallon of water, together withthe indicated ferrous salt, at 10 C., yielding in every instance asolution supersaturated with respect to .borax. The dry comprised 27.8%of NaClOs QJ QQQ (an auxiliary weed killing agent) and 72.2% of boricacid and calcined borax in proportions to give a molar ratio NazO/BzO of0.252. In all instances ferric iron was provided by supplyingthe'indicated type of ferrous salt and allowing dissolve atmosphericoxygen to oxidize the ferrous iron to ferric. The amount of ferrous ionprovided is shown as abscissa in terms of per cent on the total of othersolutes. Each solution was continuously agitated at l0 C., and thesolution phase was analyzed at 1 hour and at 24 hours. the figurerepresent the amount of borax that crystallized out of solution betweenthose two analyses, expressed in pounds per-100 lbs. of solution.

The two points at the extreme left of the figure represent solutions towhich no stabilizing agent was added. From those solutions 2.5 lbs.borax crystallized out per The ordinates in strains inst desalted startthat the assist ratio Nate/Bio; was 0.240, at ie same dosage 0r 1.51b s.of

dry mix per gallon of Water. The compositions from which Solutions B andC were prepared comprised boric acid and calcined borax only, inproportions to give repesti sadism ts bo on r i s of 0-25 and 02 3- Selutions D (Table 2) were made up with a uniform quantity of calcinedborax only, and had a. ratio of 0.5.

In Table 1 the degree of stabilization produced by the various additivesmay be judged either from the relative decrease of 3203 concentrationduring 24 hours of continuous agitation, as eornpared to thecorresponding decrease for Solutions 1 and 4 without additives; or froma similar comparison of the amount of borax crystallizing out ofsolution during that peripd. The shift in NazO/BzOs ratio is directlyattributable to the removal from the solution of a solute (borax) havinga higher ratio than the solution. It .will be noted that'solution 3 wascompletely stabilized, under the particular experimental conditionsdescribed.

In absence of stabilizing agents Solution 6 in Table 2 reached a stateof equilibrium after 24 hours. Therefore the concentration of B203;determined after that time,

represents the normal solubility at a sodium to boron ratio of 0.5. Thatvalue of the normal solubility provides a base from which the degree ofstabilization produced by the iron and aluminum may be judged. Theconcentration of B203 after 24 hours in the presence of the stabilizingagent'was more than 25% greater than the normal solubility at aNazO/BzOs ratio of 05.

Tables 1 and 2-follow:

Iab le] Solution Additive N m 13,03 Borax Crystallizod'24 Percent aQzHours, 1b./100 1h. solution 24 hours Percent on dry i 1 hour hours 100lbs. of solution during 24 hours of continuous agitation at 10 C."Although that amount of crystallization did not necessarily remove allsupersaturation fromthe solutions, it is sufficient to seriouslyinterfere with use of such solutions. Moreover, it provides a roughindication of the degree of supersaturation existing in the solutions asoriginally prepared. The progressive stabilization of thatsupersaturation as more iron was provided in the solution is clearlyshown by the progressively decreasing amount of boraxcrystallized out.When more than 0.15% ferrous ion (computed on the dry weight of theother solutes) was added, in the presence of an oxidizing agent (air),complete stabilization was obtained, and no crystallization of borax wasobserved'under the-particular experimental conditions employed. it isclear from the figure that the detailed source of ferric iron is ofminor importance" compared to the available concentration. The resultsgiven in the'figure will serve as a guide in determining the amount ofiron that should be provided'under other conditions.

In general, as the J pH of the solution increases, and as thetemperature de-' lut-ionssindicatcd as Solution Awere made with the sameAdditive Percent Solution I Peroenton Type I dry mix None Al:(SO i)a-18H O V Framin -4 one We claim: a V

l. The methpd ,of stabilizing a state of supersaturation t rar s -19'ras in a a s u is dur a h ldingperiod. of appreciable,duration; saidmethod comprisingjproducing .an aqueous solution that is substantiallysaturated with respect to borax, pro,viding in the solution astabilizingagent consisting ess'entially of an aluminum salt, theconcentration of"sa;id. stabilizing agent. being greater than-about onetenth percent on the dry Weight o t e sth 'sqlutss pts ss tiasd th h n hsolution continuously during the said holding period under conditionsfor which th:e' equilibrium solubility with respect to borax'is lessthan the existingborax concentration of the solution by an amountthat'is insuflicient, by virtue of the presence of the said stabilizingagent, to cause appreciablecrystallization of borax.

2. The methodof stabilizing a state of supersaturation with respecttoborax in aqueous solution during a holding' period. of appreciableduration; said method comprising-producing anaaqueous solutiodthat issubstantially 5 saturated with respect to borax, providing in thesolution a stabilizing agent consisting essentially of aluminum sulfate,the concentration of said stabilizing agent being greater than about onetenth per cent on the dry weight of the other solutes present, and thenholding the solution continuously during the said holding period underconditions for which the equilibrium solubility with re spect to boraxis less than the existing borax concentration preciable crystallizationof borax.

References Cited in the file of this patent UNITED STATES PATENTSKuhnert June 30, 1931

1. THE METHOD OF STABILIZING A STATE OF SUPERSATURATION WITH RESPECT TOBORAX IN AQUEOUS SOLUTION DURING A HOLD ING PERIOD OF APPRECIABLEDURATION; SAID METHOD COMPRISING PRODUCING AN AQUEOUS SOLUTION THAT ISSUBSTANTIALLY SATURATED WITH RESPECT TO BORAX, PROVIDING IN THE SOLUTIONA STABILIZING AGENT CONSISTING ESSENTIALLY OF AN ALUMINUM SALT, THECONCENTRATION OF SAID STABILIZING AGENT BEING GREATER THAN ABOUT ONETENTH PER CENT ON THE DRY WEIGHT OF THE OTHER SOLUTES PRESENT, AND THENHOLDING THE SOLUTION CONTINUOUSLY DURING THE SAID HOLDING PERIOD UNDERCONDITIONS FOR WHICH THE EQUILIBRIUM SOLUBILITY WITH RESPECT TO BORAX ISLESS THAN THE EXISTING BORAX CONCENTRATION OF THE SOLUTION BY AN AMOUNTTHAT IS INSUFFICIENT, BY VIRTUE OF THE PRESENCE OF THE SAID STABILIZINGAGENT, TO CAUSE APPRECIALBE CRYSTALLIZATION OF BORAX.